I. Field of the Invention
The present invention relates to a new method and apparatus for continuously casting certain thick sheet materials sometimes used as surface overlay, but which also can be used, without underlay, such as for a stand-alone restaurant table top. More particularly, the present invention relates to an improved method and apparatus for continuously casting polymeric materials which shrink during polymerization.
II. Description of the Prior Art
Polymeric materials such as thermosetting resins, thermoplastic resins, acrylic resins and the like are commonly used as surface covering materials. Sheets of these materials are used as decorative finishing materials in new building construction and in remodeling for such applications as bathroom vanity tops, kitchen counter tops, furniture components, restaurant tables, wall paneling and other uses. The sheets can be made of plain solid colors or can be patterned to have the appearance of marble, granite or other patterns or textural decoration. The sheets often are loaded with decorative particles to provide these characteristics and the resin composition typically includes mineral fillers such as calcium carbonate or aluminum trihydrate. It is preferred that the pattern in these materials be uniformly distributed and that the final product be flat, smooth and free of warpage, bends or wrinkles. It is also preferred that a flat surface be obtained with the minimum of sanding or machining after the sheet has been cast.
The composition of these materials typically can be a single thermosetting or thermoplastic resin or a mixture of resins such as unsaturated polyesters and acrylic resin precursors. Such resins require a catalyst and/or promoter system to initiate the process of free radical polymerization. Typical resins are characterized by having a significant exothermic reaction during polymerization and a substantial increase of density during that process. Thus, a typical mixture of 65% filler and 35% resin (a "matrix") shrinks during the polymerization process so that the cured solid composition has a density about 5-8% higher than the liquid matrix. This shrinkage presents processing problems related to the present invention which are discussed below.
A variety of prior methods have been used to achieve synthetic sheet materials having a decorative pattern. One method involves a batch process. In this process, the matrix is prepared by mixing the resins with the filler and the desired decorative particles or coloring materials and with a standard quantity of catalyst. The amount of catalyst typically recommended in the prior art is one half to two percent based on the liquid resin fraction of the matrix. This matrix is then poured or pumped into a large casting mold and sealed in the mold. The mold is then subjected to sufficient heat to begin decomposition of the catalyst, which initiates polymerization of the resin. Because the polymerization is an exothermic process, the reaction contributes to the heat of the system, leading to further catalyst decomposition and an increased rate of polymerization. This follows the typical prior art approach whereby the conductive or convective heat environment applied to the matrix is warm enough to "kick off" the polymerization reaction which then sustains itself by its own heat of reaction and actually is cooled by dissipating heat back to that environment. This process continues until substantially all of the unsaturated bonds of the resin and monomer components are consumed and the resin is cured. The mold is then opened after cooling and a panel of decorative sheet material is removed. As explained below, the present invention represents a substantial departure from the thermodynamic aspects and many other aspects of this conventional approach.
Such a batch process has significance shortcomings. It is slow and inefficient, requiring a great deal of material handling equipment. Moreover, it presents significant problems with controlling the matrix uniformity in the mold, particularly where decorative particles are used in the matrix. For example, flow patterns and convection currents in the liquid matrix which may result during pumping or during the heating and early stages of polymerization can result in nonuniformity of the decorative pattern. In addition, such a batch process presents significant curing problems if the matrix is not heated with proper uniformity and, thus, does not polymerize at the same rate throughout the mold cavity. The result can be localized shrinking which may cause cracks or tears in the final cured material or may produce residual stresses.
Several prior attempts have been made to develop a continuous casting technique as an alternative to the batch process for polymeric materials which shrink upon curing. U.S. Pat. No. 3,600,490 issued to Billingsley et al, for example, teaches that if the structure cures unevenly, as it usually does, certain areas of the mass will harden and shrink unevenly, distorting the cast product. To avoid the problem of wrinkling or tearing on the surface of the matrix during shrinking, Billingsley teaches the use of a thin film and lubricant to permit relative slippage between the shrinking matrix and the belt of the conveyor. Specifically, Billingsley teaches a process whereby the matrix rests on a layer of a film which shrinks during heating at the same rate as the curing matrix. Billingsley teaches the use of oil or a similar liquid lubricant between the film layer and the conveyor belt. In this approach, the film shrinks with the matrix and the thin film does not hang up on the conveyor belts as it shrinks.
Another continuous casting approach can be found in U.S. Pat. No. 3,988,098 issued to Kato et al. Kato teaches a dual belt system which uses the force of a confined space to control the tendencies of the matrix to distort or tear itself apart during the polymerization process. The matrix is passed through a confined space defined by upper and lower belts which force the matrix to maintain a flat rectangular cross section despite the presence of internal forces brought about by localized curing which would otherwise cause the matrix to pull apart, warp or bend.
From the foregoing it can be seen that the prior art continuous casting processes involve expensive and complicated arrangements to control the curing of the polymeric material. These prior art casting methods also pose quality problems. In conventional belt casting equipment, the liquid matrix is in thermal contact with the belts and may be heated by the conduction of heat through the belts which are enclosed in an environment of heated fluid or gas. With this arrangement, the mold or belt surface can cause initiation of a rapid accelerating reaction before curing is complete. In extreme cases this can cause local boiling, or at least, irregular cure with shrinkage stresses, cracking, ripple, craze layers, etc.
If the heating were to be done on a single supporting belt (without a top belt), the bottom of the matrix layer may polymerize before the top, causing severe warp, concave upward. Also, excessive temperature differences between the matrix center and surface could cause flow patterns to develop which result in objectionable appearance of mottling or streaks. The use of typical belt systems, particularly together with convection heat, may result in the need to limit the rate of heating, resulting in a long heating time. This in turn may require a higher matrix viscosity to reduce particle settling, and longer equipment in the case of continuous casting. Problems arise because, with the longer heating time, the viscosity of the matrix will be reduced for a longer period of time before it rapidly climbs just prior to gelling. This is illustrated in FIG. 3. The lower viscosity for an extended time will permit the mineral filler to settle and also any dense particles used for decorative effect may settle, resulting in a non-homogenous sheet both in physical properties and in appearance at a cut edge.